This invention relates to a tilt control mechanism for a chair. It relates particularly to a knee tilt control mechanism for a pedestal type of office chair.
Chair controls are mechanical devices usually mounted beneath the seat of a chair to control the tilting of a chair when a user leans back in the chair. The control mechanism usually comprises a chair control housing adapted to be mounted on a support column attached to the chair pedestal base and a chair seat support member or plate secured to the underside of the chair seat and pivotally mounted to the chair control housing. There is usually a spring or other energy storing device attached to the chair control housing which controls the rate at which the user can tilt the chair rearwardly and which returns the chair to its upright at-rest position when the user stops leaning backward.
Many of the previously known chair controls have been pivoted at a point very near the center line of the chair control housing, which usually also coincides with the center of gravity of the user seated in the chair. As a result, the tilting of the chair backwards requires very little force, but raises the front of the chair seat, creating pressure on the back of the thighs and disturbing the blood circulation of the user. These chair controls also require the user to exert considerable force through an extension of the leg and foot to maintain a tilted position for this type of chair. The result is not relaxing to the user.
More recently there have been developed knee tilt chair controls. The knee tilt chair controls function to pivot the chair seat support member or plate as near to the natural knee joint of the user as possible so that the front of the seat rises very little or not at all during the rearward tilting of the chair. With a larger portion of the user's weight positioned behind the control pivot point, little or no effort is required to recline the chair or maintain the chair in a reclined position and the feet of the user can remain flat on the floor with little effort. Some of the known knee tilt chairs reclined so easily that the user often had the feeling that the chair was falling backwards.
The knee tilt control chairs, as compared to the traditional controlled chairs, generally require a much greater force to support the user on the extended moment arm and to return the reclined user to an upright position. Springs providing such force tend to become quite large and bulky to balance the weight of the user and to return the reclining user to an upright position.
To make the most effective use of the springs used to balance the weight of the user and to return the reclined user to an upright position, the vertical force component produced by the spring should be as large as possible for all tilt positions of the chair. In the knee tilt control chair designs known prior to this invention, the spring was not being used effectively since the rate of increase of the vertical force component decreased as the chair reclined. This is because the size or magnitude of the vertical force component is a function of both the amount of torque developed by the spring and the angle of contact between the lever arm of the spring and the underside of the seat support member or plate.